1. Field of the Invention
The invention relates to a radiation emitting device, and particularly to a system and a method for adjusting the radiation delivered to an object in a radiation treatment device.
2. Description of the Related Art
Radiation-emitting devices are generally known and used, for instance as radiation therapy devices for the treatment of patients. A radiation therapy device generally comprises a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high-energy radiation beam for therapy. This high energy radiation beam can be an electron radiation or photon (X-ray) beam. During treatment, this radiation beam is trained on one zone of a patient lying in the isocenter of the gantry rotation.
In order to control the radiation emitted toward an object, a beam-shielding device such as a plate arrangement or collimator is usually provided in the trajectory of the radiation beam between the radiation source and the object. This beam-shielding device defines a field on the object to which a prescribed amount of radiation is to be delivered.
The radiation delivered to an object may be analyzed into primary and scattered components. The primary radiation is made up of the initial or original photons emitted from the radiation source, and the scattered radiation is the result of the photons scattered by the plate arrangement itself. The beam's radiation output in free space increases because of the increased collimator scatter, which is added to the primary beam. In other words, a point in the field is subjected not only to direct radiation, that is the primary component, but also to radiation that is scattered from the plate arrangement. The ratio of the radiation output in air with the scatterer to the radiation output without the scatterer for a reference field (for instance 10.times.10 cm) is commonly called the "output factor" or the collimator scatter factor. The concept and definition of the output factor are well understood in the art.
Thus, due to these scattered photons, the dose rate applied to the surface of the object changes dependent on the size of the opening in the plate arrangement, that is, on the field size. This means that the radiation emitted to the same spot, for instance in the center of the radiation beam onto the object, changes according to the size of the opening in the plate arrangement. When the plate arrangement shows only a small opening, then the accumulated dose at the same spot is less than the accumulated dose at the same spot when the opening is big.
Frequently, special filters or absorbing blocks are located in the trajectory of the radiation beam to modify its isodose distribution. A most commonly used filter is the wedge filter. This is a wedge-shaped absorbing block which causes a progressive decrease in the intensity across the beam, resulting in isodose curves that are modified relative to their normal positions. Such wedge filters are usually made of dense material, such as lead or steel, or other absorbing material.
The presence of a wedge filter decreases the output of the radiation-emitting device and this decrease must be taken into account in treatment calculations. This effect is characterized by the so-called "wedge factor", defined as the ratio of doses with and without the wedge at a point in the object along the central axis of the radiation beam. The wedge factor depends on the material, size and angle of the wedge. Wedges, and particular the wedge factor, are described in Faiz M. Khan, Ph.D, "The Physics of Radiation Therapy", Williams & Wilkins, pages 234 to 238.
The delivery of radiation by such a radiation therapy device is prescribed and approved by an oncologist. Actual operation of the radiation equipment, however, is normally done by a therapist. When the therapist administers the actual delivery of the radiation treatment as prescribed by the oncologist, the device is programmed to deliver that specific treatment. When programming the treatment, the therapist has to take into consideration the output factor and has to adjust the dose delivery based on the plate arrangement opening in order to achieve the prescribed radiation output on the surface of the object. This adjustment can be made according to known calculations, but the therapist normally has to do them manually, which can easily lead to errors. In the context of radiation therapy, a miscalculation can lead to either a dose that is too low and is ineffective, or that is too high and dangerous; a large error, for example, a misplaced decimal point, can be lethal.
U.S. Pat. No. 5,148,032 discloses a radiation treatment device in which isodose curves in the object are adjusted both by a plate arrangement, which includes at least one movable plate that is controlled during irradiation, and by varying the radiation output of the radiation beam during irradiation, so that a wide range of variations in the possible isodose curves is obtained. A wedge-shaped isodose distribution is established, for example, by moving one plate at a constant speed while simultaneously changing the radiation output of the radiation beam. In this radiation treatment device there is no physical absorbing block in the trajectory of the radiation beam, and the therapist has to take this into account.
What is needed is a method, and corresponding system, for adjusting the delivery of radiation to the object in order to make sure that the actually delivered radiation output is exactly the same as the desired radiation output, independent of the use of a wedge function.